The effects of dust, both inorganic and organic, upon the health of humanity will be considered in another chapter. Certain kinds of dust are of economic importance on account of their inflammable and explosive character when mixed with the right proportions of air. Thus the cereal dusts made in the handling and working up of grain into food products occasionally give rise to serious accidents. These occur in cereal, flour, and feed mills, grain elevators, starch and glucose factories, and on farms in connection with the use of threshing machines. During a period of ten years, 1906-1916, cereal dust explosions resulted in the loss of eighty lives and the destruction of property to a value of $2,000,000 in the United States. A study of this subject has been made by the United States Department of Agriculture, and various recommendations have been published with a view to preventing the occurrence of sparks in the neighborhood of these dangerous dusts. Coal dust in mines likewise causes numerous explosions. Preventive measures include wetting the dust, moistening the air, and powdering the walls, roof and floor of the mine with a nonexplosive rock dust, which has the effect of stifling an incipient fire or explosion. The last species of dust that we have to consider in this chapter is one that constitutes a literal blot on civilization, since the noblest cities and monuments of mankind are defaced with it. Neither are the evils of this kind of dust wholly æsthetic, for it is extremely injurious to health and enormously expensive. After enduring coal smoke as a necessary evil for generations, civilised humanity has now embarked upon a vigorous campaign for its elimination, and very encouraging results have already been achieved in many parts of the world. The war against smoke is carried on by numerous societies in Europe and America; a multitude of laws and ordinances (not all of them effective) have been enacted on the subject; it has been the occasion of international conferences and expositions; and its literature has grown so copious that a partial bibliography of the subject, published a few years ago by the Mellon Institute, of Pittsburgh, fills 164 pages.